1. Field of the Invention
The present invention relates generally to valves, and more particularly, but not by way of limitation, to an improved gate valve for controlling the flow of hydraulic fracturing fluid during well fracturing operations.
2. Description of Related Art
Several methods may be used to stimulate hydrocarbon production in old and new wells. Two common and effective methods are acidizing and hydraulic fracturing.
In general, acid treatment involves the injection of acid into an acid-soluble pay zone where the acid reacts with the rock and dissolves portions of the formation so as to enlarge existing voids and thereby increase the permeability of the zone.
Hydraulic fracturing, on the other hand, involves the injection of a fracturing fluid and propping agent into the pay zone under sufficient pressure to open existing fractures and create new fractures. The fractures are extended some distance into the formation by continued high pressure injection. Upon cessation of pumping, the fractures are held open by the propping agent which is commonly a carefully sized, silica sand.
Control and containment of injection pressures during both acidizing and hydraulic fracturing operations is critical. To this end, injection pressures are conventionally controlled with gate valves and other components such as isolation tools and blow out preventers. However, problems have been encountered when employing conventional equipment of the prior art.
For example, because of the abrasive and corrosive nature of high pressure fracturing and acidizing fluids, expensive and time consuming repairs to valves and other equipment is often necessary after each well stimulation treatment. In particular, the gate or closure member of the gate valve is galled and scratched by the high pressure, sand-carrying fluid thereby causing the valve to leak. This results in the loss of lubricant contained in the valve body to facilitate movement of the gate and to reduce wear on the gate. Loss of lubricant increases the wear on the gate and the risk that water will invade the valve body and cause the valve to freeze.
Another common problem encountered during fracturing operations is the invasion of sand or acid into the valve body. The sand becomes compacted in the top and bottom of the valve body causing the valve to be inoperable. Similarly acid reacts with the lubricant and corrodes the valve body.
All the problems discussed above, as well as others, increase operating costs and increase the chance of injury due to equipment failure. Thus, a need has long existed for an improved gate valve which has a hard sealing surface resistant to galling and scratching and which is designed to prevent the invasion of proppant into the valve body and the loss of lubricant, while also being inexpensive to manufacture, easy to maintain and convenient to operate. It is to such a gate valve that the present invention is directed.